1. Field of the Invention
The present invention relates in general to transmissions for use in motor vehicles, and more particularly to continuously variable transmissions of a friction roller type.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional continuously variable transmission of the friction roller type will be described, which is described in Japanese Patent First Provisional Publication No. 4-29659.
The transmission shown in the publication comprises generally a first continuously variable transmission mechanism which includes a first input disc, a first output disc, a pair of first friction rollers operatively interposed between the first input and output discs, a pair of first roller supporting members which rotatably support through respective eccentric shafts the first friction rollers and are rotatable about and slidable along respective rotation axes and a pair of first hydraulic power devices which move the first roller supporting members along the respective rotation axes and thus adjust the angular positions of the first friction rollers and a second continuously variable transmission mechanism which includes a second input disc, a second output disc, a pair of second friction rollers operatively interposed between the second input and output discs, a pair of second roller supporting members which rotatably support through respective eccentric shafts the second friction rollers and are rotatable and slidable along respective rotation axes and a pair of second hydraulic power devices which move the second roller supporting members along the respective rotation axes and thus adjust the angular positions of the second friction rollers.
The transmission further comprises a speed control valve which adjusts the hydraulic pressure fed to the first and second paired hydraulic power devices, and a thrust cam device installed at a back side of the first input disc to bias the first and second input discs toward the first and second output discs respectively.
The first continuously variable transmission mechanism is equipped with a cam mechanism which is operatively interposed between the speed control valve and one of the first roller supporting members. Due to provision of the cam mechanism, a feedback control for the first continuously variable transmission mechanism is carried out in which the existing condition of the first transmission mechanism is fed back to the speed control valve which adjusts the angular positions which the first and second friction rollers assume.
However, due to its inherent construction, the above-mentioned conventional transmission has the following drawbacks.
That is, the biasing force produced by the thrust cam device is not evenly applied to the first and second input discs of the first and second continuously variable transmission mechanisms. That is, the biasing force transmitted to the second input disc is somewhat smaller than that to the first input disc. This is because the biasing force for the second input disc should be transmitted thereto through an input shaft which has various bearings mounted thereto, which bearings cause non-negligible friction resistance against the axial movement of the input shaft. As is known, such insufficient biasing force tends to cause an unstable operation of the second continuously variable transmission mechanism due to slippage of the friction rollers or the like.